JP7421161B2 - Method for manufacturing alkali-free glass substrate and alkali-free glass substrate - Google Patents

Description

The present invention relates to an alkali-free glass substrate, and more particularly to an alkali-free glass substrate suitable for a display including a thin film transistor (TFT) having a low temperature p-Si (LTPS) film.

A glass substrate is generally used as a support substrate for flat panel displays. An electric circuit pattern such as a TFT is formed on the surface of this glass substrate. Therefore, for this type of glass substrate, an alkali-free glass substrate that does not substantially contain an alkali metal component is used so as not to adversely affect TFTs and the like.

Further, the glass substrate is exposed to a high temperature atmosphere during the process of forming an electrical circuit pattern such as a thin film forming process or a thin film patterning process. When a glass substrate is exposed to a high-temperature atmosphere, structural relaxation of the glass progresses, so that the volume of the glass substrate contracts (hereinafter, this contraction of the glass is referred to as "thermal contraction"). If heat shrinkage occurs in the glass substrate during the process of forming the electrical circuit pattern, the shape and dimensions of the electrical circuit pattern formed on the glass substrate will deviate from the designed values, making it difficult to obtain a flat panel display with the desired electrical performance. It becomes difficult. Therefore, it is desired that a glass substrate, such as a glass substrate for a flat panel display, on which a thin film pattern such as an electric circuit pattern is formed on the surface, has a low thermal shrinkage rate.

In particular, in the case of glass substrates for high-definition displays equipped with TFTs having low-temperature polysilicon films, when forming the low-temperature polysilicon films, they are exposed to extremely high temperature atmospheres of, for example, 450°C to 600°C, causing thermal shrinkage. However, since the electrical circuit pattern is highly precise, if thermal contraction occurs, it becomes difficult to obtain the desired electrical performance. Therefore, it is strongly desired that glass substrates used in such applications have a very low thermal shrinkage rate.

Incidentally, as methods for forming glass substrates used in flat panel displays and the like, there are known methods such as a float method and a down-draw method represented by an overflow down-draw method.

The float method is to flow molten glass onto a float bath filled with molten tin, stretch it horizontally to form a glass ribbon, and then slowly cool the glass ribbon in an annealing furnace installed downstream of the float bath. This is a method of molding a glass substrate. In the float method, since the glass ribbon is conveyed in the horizontal direction, it is easy to lengthen the lehr. For this reason, it is easy to make the cooling rate of the glass ribbon in the slow cooling furnace sufficiently low. Therefore, the float method has the advantage that it is easy to obtain a glass substrate with a small thermal shrinkage rate.

However, the disadvantage of the float method is that it is difficult to mold thin glass substrates, and that the surface of the glass substrate must be polished to remove tin adhering to the surface of the glass substrate after molding. There are disadvantages.

On the other hand, the down-draw method is a method of drawing molten glass downward to form it into a plate shape. The overflow down-draw method, which is a type of down-draw method, is a method of forming a glass ribbon by stretching molten glass overflowing from both sides of a forming body having a generally wedge-shaped cross section downward. The molten glass overflowing from both sides of the molded body flows down along both sides of the molded body and merges below the molded body. Therefore, in the overflow down-draw method, the surface of the glass ribbon does not come into contact with anything other than air, and is formed by surface tension, so there is no need to polish the surface after molding, and the surface is free from foreign matter. can obtain a flat glass substrate. Further, the overflow down-draw method has the advantage that it is easy to mold a thin glass substrate.

On the other hand, in the down-draw method, molten glass flows downward from the molded body. If a long lehr is to be placed below the molded product, the molded product must be placed at a high location. However, in practice, there are restrictions on the height at which the molded product can be placed due to restrictions on the height of the factory ceiling. That is, in the down-draw method, there are restrictions on the length of the lehr, and it may be difficult to arrange a sufficiently long lehr. When the length of the lehr is short, the cooling rate of the glass ribbon becomes high, making it difficult to form a glass substrate with a small thermal shrinkage rate.

Therefore, it has been proposed to increase the strain point of glass to reduce its thermal shrinkage rate. For example, Patent Document 1 discloses an alkali-free glass composition with a high strain point. The same document also states that the lower the β-OH value, which represents the amount of water in the glass, the higher the strain point.

Japanese Patent Application Publication No. 2013-151407

As shown in FIG. 1, the higher the strain point, the lower the thermal contraction rate. However, glass whose composition is designed to have a high strain point has a high viscosity, so there is a problem that bubble breakage is difficult and it is difficult to obtain a glass with excellent bubble quality.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an alkali-free glass substrate having a high strain point and excellent bubble quality, and a method for manufacturing the same.

Since alkali-free glass does not substantially contain alkali metal oxides, it is known that raw material batches are difficult to dissolve. Therefore, it is common to carry out the melting step for melting the raw material batch at high temperatures. The present inventors have proposed the present invention by paying attention to the fact that when the melting process is performed at a high temperature, gas generated from the fining agent is less likely to be generated in the subsequent fining process.

That is, in the method for producing an alkali-free glass substrate of the present invention, the glass composition, in mass %, is 50 to 70% SiO ₂ , 15 to 25% Al ₂ O ₃ , more than 4.5 to 12% B ₂ O ₃ , MgO 0-10%, CaO 0-15%, SrO 0-10%, BaO 0-15%, ZnO 0-5%, ZrO ₂ 0-5%, TiO ₂ 0-5%, P ₂ O ₅ 0-15 %, a batch preparation step of preparing a raw material batch to obtain an alkali-free glass containing 0 to 0.5% of SnO ₂ , a melting step of melting the prepared raw material batch, and a fining step of clarifying the molten glass. and a forming step of forming the clarified glass into a plate shape, characterized in that the raw material batch is melted so that the temperature at which the resulting glass starts to expand its bubble diameter is lower than the maximum temperature in the fining step. .

Here, "alkali-free glass" refers to glass to which no alkali metal oxide components are intentionally added, and specifically, alkali metal oxides (Li ₂ O, Na ₂ O, and K ₂ O) content is 3000 ppm (mass) or less. Note that the content of alkali metal oxide in the glass composition is preferably 2000 ppm or less. "Bubble diameter expansion start temperature" means a temperature specified by the following method. First, the obtained glass is crushed and classified, and then held at 1500° C. for 10 minutes. Thereafter, the temperature was raised from 1500°C at a heating rate of 2°C/min, and the behavior of bubbles in the glass melt was observed. Select three or more arbitrary bubbles with a diameter of 100 μm or less, measure their bubble diameter at every 10°C, and define the temperature at which the bubble diameter increases by 50 μm or more compared to the bubble diameter of 1500°C as the bubble diameter expansion start temperature. do.

In the present invention, since the B ₂ O ₃ content of the glass composition used is low, it is possible to obtain a glass substrate with a high strain point. However, glasses with high strain points generally have high viscosity, making it difficult to achieve high foam quality. Therefore, in the method of the present invention, the raw material batch is melted such that the temperature at which bubble diameter expansion starts in the resulting glass is lower than the maximum temperature in the fining step. The "bubble diameter expansion start temperature" defined in the present invention is the temperature at which the bubbles in the glass become large enough to float. In the fining process, the bubbles in the glass sufficiently expand and float easily, making it possible to obtain a glass with excellent bubble quality.

In addition, the method for producing an alkali-free glass substrate of the present invention includes, in terms of glass composition, SiO ₂ 50 to 70%, Al ₂ O ₃ 15 to 25%, B ₂ O ₃ more than 4.5 to 12%, MgO 0-10%, CaO 0-15%, SrO 0-10%, BaO 0-15%, ZnO 0-5%, ZrO ₂ 0-5%, TiO ₂ 0-5%, P ₂ O ₅ 0-15 %, a batch preparation step of preparing a raw material batch to obtain an alkali-free glass containing 0 to 0.5% of SnO ₂ , a melting step of melting the prepared raw material batch, and a fining step of clarifying the molten glass. , a forming process of forming the clarified glass into a plate shape, and an evaluation process of evaluating the bubble quality of the obtained glass, and adjusting the temperature at which bubble diameter expansion starts based on the bubble quality of the obtained glass. It is characterized by

In addition to the above-mentioned effects, the manufacturing method of the present invention that adopts the above configuration makes it easy to correct even if the bubble diameter expansion start temperature temporarily becomes higher than the maximum temperature in the fining process. become. Therefore, glass with excellent bubble quality can be stably obtained.

In the manufacturing method of the present invention, it is preferable to melt the raw material batch so that the temperature at which the resulting glass begins to expand in bubble diameter is 1520 to 1680°C.

If the above configuration is adopted, it becomes easy to lower the temperature at which the bubble diameter expansion of the glass begins to occur than the maximum temperature in the fining step, and it becomes easier to obtain glass with excellent bubble quality. Further, it is possible to easily avoid a situation where the temperature in the fining step becomes too high.

In the manufacturing method of the present invention, electric melting is preferred. Here, "electric melting" is a melting method in which electricity is passed through the glass, and the Joule heat generated thereby is used to heat and melt the glass. Note that supplementary use of radiant heating by a heater or burner is not excluded.

By employing the above configuration, it is possible to suppress an increase in moisture in the atmosphere. As a result, it becomes possible to significantly suppress the supply of moisture from the atmosphere to the glass, making it easier to manufacture glass with a high strain point. Furthermore, since the glass melt is heated using the heat generated by the glass itself (Joule heat), the glass can be heated efficiently. Therefore, it becomes possible to melt the raw material batch at a relatively low temperature, and it becomes easier to lower the temperature at which bubble diameter expansion starts.

In the manufacturing method of the present invention, it is preferable not to use radiant heating using burner combustion. "Radiant heating by burner combustion is not used in combination" means that radiant heating by burner combustion is not used at all during normal production, but does not exclude the use of burners at the start of production (at the time of temperature rise). Furthermore, this does not exclude the use of radiant heating using a heater at the time of starting up production or during normal production. Note that the time of production start-up refers to the period from when the raw material batch is melted into a glass melt until it can be heated with electricity.

If the above configuration is adopted, the amount of moisture contained in the atmosphere within the melting furnace is extremely reduced, and the amount of moisture supplied from the atmosphere into the glass can be significantly reduced. As a result, it becomes possible to produce glass with extremely low water content. In addition, the equipment necessary for combustion heating, such as burners, flues, fuel tanks, fuel supply routes, air supply equipment (for air combustion), oxygen generators (for oxygen combustion), exhaust gas treatment equipment, dust collectors, etc. This is not necessary or can be greatly simplified, making it possible to make the melting furnace more compact and reduce equipment costs. Furthermore, it is possible to melt the raw material batch at a low temperature, making it easier to lower the temperature at which bubble diameter expansion begins.

In the production method of the present invention, it is preferable to add chloride to the raw material batch.

Chloride has the effect of lowering the moisture content in glass. When the water content in the glass decreases, the strain point of the glass increases. Therefore, by adopting the above configuration, it becomes easy to manufacture glass with a high strain point.

In the production method of the present invention, it is preferable to use boric anhydride as at least a part of the glass raw material serving as the boron source.

By employing the above configuration, it becomes possible to reduce the moisture content of the resulting glass.

In the production method of the present invention, it is preferable that the raw material batch does not contain a hydroxide raw material.

By employing the above configuration, it becomes possible to further reduce the moisture content of the resulting glass.

The manufacturing method of the present invention is a method for manufacturing an alkali-free glass substrate by adding glass cullet to a raw material batch, wherein at least a part of the glass cullet has a β-OH value of 0.4/mm or less. Preferably, a glass cullet made of glass is used. Here, the term "glass cullet" refers to defective glass produced during glass manufacturing or recycled glass collected from the market. "β-OH value" refers to a value obtained by measuring the transmittance of glass using FT-IR and using the following formula.

β-OH value = (1/X)log(T1/T2)
X: Glass thickness (mm)
T1: Transmittance (%) at reference wavelength 3846 cm ^-1
T2: Minimum transmittance (%) near hydroxyl absorption wavelength 3600 cm ^-1
Since alkali-free glass has a high volume resistivity, it tends to be more difficult to melt than glass containing alkali. Therefore, if the above configuration is adopted, it becomes easier to melt the glass, and it becomes possible to further reduce the moisture content of the obtained glass.

In the manufacturing method of the present invention, it is preferable to adjust the glass raw materials and/or melting conditions so that the β-OH value of the glass obtained is 0.3/mm or less.

By employing the above configuration, it becomes easy to obtain glass with a high strain point and high thermal shrinkage rate.

In the manufacturing method of the present invention, it is preferable that the strain point of the glass obtained is higher than 650°C. Here, the "strain point" is a value measured based on the method of ASTM C336-71.

By employing the above configuration, it is possible to obtain glass with an extremely low thermal shrinkage rate.

In the manufacturing method of the present invention, it is preferable that the heat shrinkage rate of the glass obtained is 30 ppm or less. Here, "thermal shrinkage rate" refers to heat treatment under the condition that the glass is heated from room temperature to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then cooled at a rate of 5°C/min. This is the value measured later.

By employing the above configuration, a glass substrate suitable for forming a low-temperature polysilicon TFT can be obtained.

Further, the method for producing an alkali-free glass substrate of the present invention includes a batch preparation step of preparing a raw material batch to obtain an aluminosilicate-based alkali-free glass having a B ₂ O ₃ content of more than 4.5% by mass to 12%. , a melting process for melting the prepared raw material batch, a fining process for fining the molten glass, and a forming process for forming the clarified glass into a plate shape. It is characterized by melting the raw material batch so that the temperature at which bubble diameter expansion begins is low. Here, "aluminosilicate" refers to a glass composition system containing SiO ₂ and Al ₂ O ₃ as main components. More specifically, it refers to a glass composition containing 50 to 80% SiO ₂ and 15 to 30% Al ₂ O ₃ by mass.

Furthermore, the alkali-free glass substrate of the present invention has a glass composition, in mass %, of SiO ₂ 50 to 70%, Al ₂ O ₃ 15 to 25%, B ₂ O ₃ more than 4.5 to 12%, MgO 0 to 10%. %, CaO 0-15%, SrO 0-10%, BaO 0-15%, ZnO 0-5%, ZrO ₂ 0-5%, TiO ₂ 0-5%, P ₂ O ₅ 0-15%, SnO ₂₀ to 0.5%, and has a bubble diameter expansion starting temperature of 1520 to 1680°C.

In the alkali-free glass substrate of the present invention, it is preferable that the β-OH value is 0.3/mm or less.

In the alkali-free glass substrate of the present invention, it is preferable that the strain point is higher than 650°C.

In the alkali-free glass substrate of the present invention, it is preferable that the thermal shrinkage rate is 30 ppm or less.

The alkali-free glass substrate of the present invention is preferably used as a glass substrate on which a low-temperature p-SiTFT is formed.

Low-temperature polysilicon TFTs are heat-treated at high temperatures (nearly 450 to 600° C.) when formed on a substrate, and the circuit pattern becomes finer. Therefore, glass substrates used for this type of application are required to have particularly low thermal shrinkage. Therefore, the advantage of employing the glass substrate of the present invention, which has a high strain point, is extremely large.

Further, the alkali-free glass substrate of the present invention is an aluminosilicate-based alkali-free glass with a B ₂ O ₃ content of more than 4.5 to 12% by mass, and has a bubble diameter expansion starting temperature of 1520 to 1680°C. Features.

It is a graph showing the relationship between the strain point and thermal shrinkage rate of glass. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing a schematic configuration of glass manufacturing equipment for carrying out the manufacturing method of the present invention. FIG. 3 is a plan view for explaining a procedure for measuring the thermal shrinkage rate of a glass substrate.

Hereinafter, the method for manufacturing an alkali-free glass substrate of the present invention will be described in detail.

The method of the present invention is a method for continuously manufacturing an alkali-free glass substrate, including a batch preparation step of preparing a raw material batch, a melting step of melting the prepared raw material batch, and a fining step of clarifying the molten glass. and a molding step of molding the clarified glass. Each step will be explained in detail below.

(1) Batch Preparation Step First, glass raw materials are prepared so that the desired alkali-free glass can be obtained. For example, glass raw materials can be prepared to be alkali-free glass with a strain point of 650° C. or higher. Further, the glass raw material can be prepared so as to be an aluminosilicate-based alkali-free glass having a B ₂ O ₃ content of more than 4.5 to 12% by mass. More specifically, the glass composition, in mass %, is SiO ₂ 50 to 70%, Al ₂ O ₃ 15 to 25%, B ₂ O ₃ more than 4.5 to 12%, MgO 0 to 10%, CaO 0 ~15%, SrO 0-10%, BaO 0-15%, ZnO 0-5%, ZrO ₂ 0-5%, TiO ₂ 0-5%, P ₂ O ₅ 0-15%, SnO ₂ 0-0 It is preferable to prepare the glass raw material so that it becomes an alkali-free glass containing .5%. The reason why the content of each component was regulated as described above will be explained below. Note that % in the description of each component below refers to mass % unless otherwise specified. Further, the raw materials to be used will be described later.

SiO ₂ is a component that forms the skeleton of glass. The content of SiO ₂ is preferably 50-70%, 50-69%, 50-68%, 51-67%, 51-66%, 51.5-65%, particularly 52-64%. If the content of SiO ₂ is too low, the density becomes too high and the acid resistance tends to decrease. On the other hand, if the content of SiO ₂ is too large, the high temperature viscosity will increase and the meltability will tend to decrease. Furthermore, devitrified crystals such as cristobalite tend to precipitate, and the liquidus temperature tends to rise.

Al ₂ O ₃ is a component that forms the skeleton of glass, a component that increases the strain point and Young's modulus, and a component that suppresses phase separation. The content of Al ₂ O ₃ is 15-25%, 15-24%, 15-23%, 15.5-22%, 16-21.5%, 16.5-21%, especially 17-20.5 % is preferable. If the content of Al ₂ O ₃ is too small, the strain point and Young's modulus tend to decrease, and the glass tends to undergo phase separation. On the other hand, if the content of Al ₂ O ₃ is too large, devitrification crystals such as mullite and anorthite are likely to precipitate, and the liquidus temperature is likely to rise.

B ₂ O ₃ is a component that increases meltability and devitrification resistance. The content of B ₂ O ₃ is more than 4.5 to 12%, more than 4.5 to 11%, more than 4.5 to 10%, more than 4.5 to 9.5%, more than 4.5 to 9%, More than 4.5 to 8.5%, More than 4.5 to 8%, 4.6 to 7.5%, 4.8 to 7.2%, 5 to 7%, 5 to 6.7%, Especially 5 It is preferably 6.5%. If the content of B ₂ O ₃ is too small, meltability and devitrification resistance tend to decrease, and resistance to hydrofluoric acid-based chemicals tends to decrease. On the other hand, if the content of B ₂ O ₃ is too large, the strain point and Young's modulus tend to decrease. Also, the amount of water brought in from the batch increases.

MgO is a component that lowers high-temperature viscosity and increases meltability, and among alkaline earth metal oxides, it is a component that significantly increases Young's modulus. The content of MgO is preferably 0 to 10%, 0 to 9%, 1 to 8%, 1 to 7%, 1.5 to 7.5%, particularly 2 to 6%. If the MgO content is too low, meltability and Young's modulus tend to decrease. On the other hand, if the content of MgO is too large, the devitrification resistance tends to decrease and the strain point tends to decrease.

CaO is a component that lowers high temperature viscosity and significantly increases meltability without lowering strain point. Furthermore, among alkaline earth metal oxides, the raw material to be introduced is relatively inexpensive, so it is a component that reduces raw material costs. The content of CaO is 0-15%, 0-13%, 0-12%, 0-11%, 0-10.5%, 0-10%, 1-10%, 2-9%, 3-8 %, 3.5 to 7%, particularly 4 to 6%. If the CaO content is too low, it will be difficult to enjoy the above effects. On the other hand, if the content of CaO is too large, the glass tends to devitrify and the coefficient of thermal expansion tends to increase.

SrO is a component that suppresses phase separation and improves devitrification resistance. Furthermore, it is a component that lowers high temperature viscosity and increases meltability without lowering strain point. It is also a component that suppresses an increase in liquidus temperature. The SrO content is preferably 0 to 10%, 0 to 9%, 0 to 8%, 0.5 to 7.5%, particularly 0.5 to 7%. If the SrO content is too low, it will be difficult to enjoy the above effects. On the other hand, if the SrO content is too high, strontium silicate-based devitrification crystals tend to precipitate, and the devitrification resistance tends to decrease.

BaO is a component that significantly improves devitrification resistance. The content of BaO is preferably 0 to 15%, 0 to 14%, 0 to 13%, 0 to 12%, particularly 0.5 to 10.5%. If the BaO content is too low, it will be difficult to enjoy the above effects. On the other hand, if the BaO content is too high, the density becomes too high and the meltability tends to decrease. Furthermore, devitrified crystals containing BaO tend to precipitate, and the liquidus temperature tends to rise.

ZnO is a component that increases meltability. However, when a large amount of ZnO is contained, the glass tends to devitrify and the strain point tends to decrease. The content of ZnO is preferably 0 to 5%, 0 to 4%, 0 to 3%, particularly 0 to 2%.

ZrO ₂ is a component that enhances chemical durability, but when a large amount of ZrO ₂ is contained, devitrification of ZrSiO ₄ tends to occur. The content of ZrO ₂ is preferably 0-5%, 0-4%, 0-3%, 0-2%, particularly 0-0.1%.

TiO ₂ is a component that lowers high temperature viscosity and increases meltability. It is also a component that suppresses solarization. However, when a large amount of TiO ₂ is contained, the glass becomes colored and the transmittance tends to decrease. The content of TiO ₂ is preferably 0-5%, 0-4%, 0-3%, 0-2%, particularly 0-0.1%.

P ₂ O ₅ is a component that increases the strain point and is also a component that can suppress the precipitation of devitrified crystals of alkaline earth aluminosilicate such as anorthite. However, when a large amount of P ₂ O ₅ is contained, the glass tends to undergo phase separation. The content of P ₂ O ₅ is preferably 0-15%, 0-13%, 0-12%, 0-11%, 0-10%, 0-9%, 0-8%, 0-7% , 0-6%, especially 0-5%.

SnO ₂ is a component that has a good clarification effect in a high temperature range, is a component that increases the strain point, and is a component that reduces high temperature viscosity. It also has the advantage of not corroding the molybdenum electrode. The content of SnO ₂ is 0-0.5%, 0.001-0.5%, 0.001-0.45%, 0.001-0.4%, 0.01-0.35%, 0 .1 to 0.3%, particularly 0.15 to 0.3%. If the content of SnO ₂ is too large, devitrified crystals of SnO ₂ are likely to precipitate, and devitrified crystals of ZrO ₂ are likely to be precipitated. Note that if the content of SnO ₂ is less than 0.001%, it becomes difficult to enjoy the above effects.

In addition to the above components, other components such as Cl and F can be contained in a total amount of 10% or less, particularly 5% or less. However, it is preferable that As ₂ O ₃ and Sb ₂ O ₃ are not substantially contained from the viewpoint of environment and prevention of electrode erosion. Here, "substantially not containing" means that glass raw materials and glass cullet containing these components are not intentionally added to the glass batch. More specifically, it means that arsenic is 50 ppm or less as As ₂ O ₃ and antimony is 50 ppm or less as Sb ₂ O ₃ in the resulting glass.

Next, the glass raw materials constituting the batch will be explained. Note that % in the description of each raw material below refers to mass % unless otherwise specified.

Silica sand (SiO ₂ ) or the like can be used as a silicon source.

Alumina (Al ₂ O ₃ ), aluminum hydroxide (Al(OH) ₃ ), etc. can be used as the aluminum source. Note that since aluminum hydroxide contains crystallization water, if the proportion of aluminum hydroxide used is large, it becomes difficult to reduce the water content of the glass. Therefore, it is preferable to use as little aluminum hydroxide as possible. Specifically, the proportion of aluminum hydroxide used can be 50% or less, 40% or less _, 30% or less, 20% or less, or 10% or less with respect to 100% of the aluminum source (in terms of Al ₂ O 3). Preferably, preferably not used.

Orthoboric acid (H ₃ BO ₃ ) and boric anhydride (B ₂ O ₃ ) can be used as the boron source. Since orthoboric acid contains water of crystallization, if it is used in a large proportion, it becomes difficult to reduce the water content of the glass. For this reason, it is preferable to use as high a proportion of boric anhydride as possible. Specifically, it is preferable that the proportion of boric anhydride used is 50% or more, 70% or more, or 90% or more with respect to 100% of the boron source (in terms of B ₂ O ₃ ). It is desirable to do so.

Alkaline earth metal sources include calcium carbonate (CaCO ₃ ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ₂ ), barium carbonate (BaCO ₃ ), barium nitrate (Ba(NO ₃ ) ₂ ), Strontium carbonate (SrCO ₃ ), strontium nitrate (Sr(NO ₃ ) ₂ ), etc. can be used. Note that since magnesium hydroxide contains water of crystallization, it becomes difficult to reduce the water content of the glass when the proportion of magnesium hydroxide used is large. Therefore, it is preferable to avoid using magnesium hydroxide as much as possible. Specifically, it is preferable that the usage ratio of magnesium hydroxide is 50% or less, 40% or less, 30% or less, 20% or less, or 10% or less with respect to 100% of the magnesium source (MgO equivalent). It is recommended not to use it.

Zinc oxide (ZnO) or the like can be used as a zinc source.

Zircon (ZrSiO ₄ ) or the like can be used as the zirconia source. Note that when a Zr-containing refractory such as zirconia electrocast refractory or dense zircon is used as the refractory constituting the melting furnace, zirconia components may be eluted from the refractory. These eluted components may also be used as a zirconia source.

Titanium oxide (TiO ₂ ) or the like can be used as a titanium source.

As a phosphorus source, aluminum metaphosphate (Al(PO ₃ ) ₃ ), magnesium pyrophosphate (Mg ₂ P ₂ O ₇ ), etc. can be used.

Tin oxide (SnO ₂ ) or the like can be used as a tin source. When using tin oxide, it is preferable to use tin oxide having an average particle diameter D ₅₀ in the range of 0.3 to 50 μm, 2 to 50 μm, particularly 5 to 50 μm. If the average particle size D ₅₀ of the tin oxide powder is small, agglomeration between particles will occur, making clogging in a compounding plant more likely to occur. On the other hand, if the average particle diameter D ₅₀ of the tin oxide powder is large, the dissolution reaction of the tin oxide powder into the glass melt is delayed, and the clarification of the melt does not proceed. As a result, oxygen gas cannot be sufficiently released at an appropriate time during glass melting, and bubbles tend to remain in the glass product, making it difficult to obtain a product with excellent bubble quality. Moreover, undissolved spots of SnO ₂ crystals are likely to appear in the glass product.

In addition, from the viewpoint of limiting the water content of the glass, the proportion of hydroxide raw materials such as aluminum hydroxide and magnesium hydroxide used is preferably 5% or less, 3% or less relative to the batch, and should not be used if possible. is desirable.

Furthermore, in the present invention, chloride may be included in the batch. Chloride acts as a dehydrating agent that significantly reduces the water content of the glass. It also has the effect of promoting the action of the tin compound, which is a clarifying agent. Furthermore, chloride decomposes and evaporates in a temperature range of 1200° C. or higher to generate clear gas, and its stirring effect suppresses the formation of a heterogeneous layer. In addition, chloride has the effect of taking in and dissolving silica raw materials such as silica sand during its decomposition. As the chloride, for example, alkaline earth metal chlorides such as strontium chloride, aluminum chloride, etc. can be used.

In the present invention, it is desirable that the batch be substantially free of arsenic compounds and antimony compounds. If these components are contained, they will corrode the molybdenum electrode, making it difficult to perform stable electric melting over a long period of time. Moreover, these components are environmentally undesirable.

In the present invention, it is preferable to use glass cullet in addition to the above-mentioned glass raw materials. When using glass cullet, the ratio of glass cullet to the total amount of the raw material batch is preferably 1% by mass or more, 5% by mass or more, particularly 10% by mass or more. Although there is no upper limit to the usage ratio of glass cullet, it is preferably 50% by mass or less, 40% by mass or less, particularly 30% by mass or less. In addition, at least a part of the glass cullet used has a β-OH value of 0.4/mm or less, 0.35/mm or less, 0.3/mm or less, 0.25/m or less, or 0.2/mm or less. , 0.18/mm or less, 0.17/mm or less, 0.16/m or less, particularly 0.15/mm or less, and it is desirable to use a low-moisture glass cullet made of glass. Note that the lower limit of the β-OH value of the low moisture glass cullet is not particularly limited, but realistically it is 0.01/mm or more.

The amount of low moisture glass cullet used is preferably 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more based on the total amount of glass cullet used, especially the total amount. It is desirable to use low moisture glass cullet. If the β-OH value of the low-moisture glass cullet is not sufficiently low, or if the proportion of the low-moisture glass cullet used is small, the effect of lowering the β-OH value of the resulting glass will be reduced.

Note that the glass raw material, the glass cullet, or the raw material batch prepared from these may contain water. It may also absorb moisture from the atmosphere during storage. Therefore, in the present invention, it is preferable to introduce dry air into a raw material silo for weighing and supplying individual glass raw materials, a furnace silo for charging a prepared raw material batch into a melting kiln, and the like.

(2) Melting process Next, the prepared raw material batch is melted so that the bubble diameter expansion start temperature is lower than the maximum temperature in the clarification process. Note that in order to lower the bubble diameter expansion start temperature, the maximum temperature within the melting furnace may be lowered. For example, in the case of electric melting without burner combustion, the temperature can be adjusted by changing the temperature near the bottom of the electrode in the melting furnace.

A melting furnace that can be heated with radiant heat from burner combustion and Joule heat generated by electricity flowing between electrodes is used to melt the raw material batch. In particular, it is preferable to use a melting furnace capable of electric melting.

A melting furnace capable of electric melting has multiple electrodes made of molybdenum, platinum, tin, etc. By applying electricity between these electrodes, electricity is passed through the glass melt, and the Joule heat is generated. Continuously melts glass. Incidentally, radiant heating using a heater or burner may be used as an auxiliary method, but from the viewpoint of reducing the β-OH value of the glass, it is preferable to use complete electric melting without using a burner. When heating with a burner, moisture generated by combustion is incorporated into the glass, making it difficult to sufficiently reduce the moisture content of the glass.

As the electrode, it is preferable to use a molybdenum electrode. Since molybdenum electrodes have a high degree of freedom in placement location and electrode shape, it is possible to adopt the optimal electrode placement and electrode shape even with alkali-free glass, which is difficult to conduct electricity, and facilitates electrical heating. The electrode shape is preferably rod-shaped. If it is rod-shaped, it is possible to arrange a desired number of electrodes at any position on the side wall surface or bottom wall surface of the melting furnace while maintaining a desired distance between the electrodes. Regarding the arrangement of electrodes, it is desirable to arrange a plurality of pairs of electrodes on the wall surface (side wall surface, bottom wall surface, etc.) of the melting furnace, particularly on the bottom wall surface, with short distances between the electrodes. Note that if the glass contains arsenic components or antimony components, molybdenum electrodes cannot be used for the reasons mentioned above, and it is necessary to use tin electrodes that are not corroded by these components instead. However, tin electrodes have very low flexibility in placement and electrode shape, making it difficult to electrically melt alkali-free glass.

A raw material batch put into a melting furnace is melted by radiant heat or Joule heat, and becomes a glass melt (molten glass). When chloride is contained in the raw material batch, the chloride decomposes and evaporates to remove moisture in the glass into the atmosphere, reducing the β-OH value of the glass. Further, polyvalent oxides such as tin compounds contained in the raw material batch dissolve in the glass melt and act as a clarifying agent. For example, the tin component releases oxygen bubbles during the heating process. The released oxygen bubbles expand and float the bubbles contained in the glass melt, and are removed from the glass. Furthermore, the tin component absorbs oxygen bubbles during the temperature cooling process, thereby eliminating any bubbles remaining in the glass.

(3) Clarifying process Next, the temperature of the molten glass is raised and the glass is refined. The clarification step may be performed in an independent clarification tank, or may be performed in a downstream part of the melting kiln.

The molten glass to be subjected to the fining process is one that has been melted in the melting process such that the bubble diameter expansion start temperature is lower than the maximum temperature in the fining process (hereinafter referred to as maximum fining temperature). When the temperature of the glass melt reaches a higher temperature than when it is melted, oxygen bubbles are released from the fining agent component due to the above-mentioned reaction, and the bubbles contained in the glass melt can be expanded and floated to be removed from the glass. At this time, the larger the difference between the temperature during melting and the temperature during fining, the higher the fining effect. Therefore, it is desirable to lower the melting temperature as much as possible. The bubble diameter expansion starting temperature is a guideline for the temperature at the time of melting.

The bubble diameter expansion start temperature can be determined by remelting the obtained glass according to the following procedure. First, the obtained glass is crushed and classified into 2.0 to 5.6 mm. 15 g of classified glass is placed in a quartz tube and held at 1500° C. for 10 minutes. Thereafter, the temperature was raised from 1500°C at a heating rate of 2°C/min, and the behavior of bubbles in the glass melt was observed. Using a video or an observation image extracted from the video, measure the temperature at which bubble diameter expansion begins as follows. Three or more arbitrary bubbles with a diameter of 100 μm or less are selected, and their bubble diameters are measured every 10°C. The temperature at which the bubble diameter expands by 50 μm or more relative to the bubble diameter of 1500° C. is defined as the bubble diameter expansion start temperature. If the selected bubble absorbs other bubbles and the bubble diameter increases, it is necessary to select the bubbles again. Note that in consideration of the burden on the equipment, it is desirable that the temperature be raised to 1680°C, particularly 1650°C.

The bubble diameter expansion starting temperature is preferably 1520 to 1680°C, 1530 to 1670°C, 1530 to 1660°C, 1530 to 1640°C, 1530 to 1630°C, 1530 to 1625°C, particularly 1530 to 1620°C. Further, the maximum temperature in the fining step is preferably 1540 to 1640°C, 1540 to 1635°C, particularly 1540 to 1630°C. Further, the temperature difference between the bubble diameter expansion start temperature and the maximum fining temperature is preferably 15°C or more, 20°C or more, particularly 25°C or more. When the temperature difference between the bubble diameter expansion start temperature and the maximum fining temperature is large, the fining effect becomes large. Furthermore, even when the melting conditions change, the number of bubbles in the obtained plate glass is unlikely to increase. Although the upper limit of the temperature difference between the bubble diameter expansion start temperature and the maximum fining temperature is not limited, it is realistically preferably 200°C or less, 170°C or less, particularly 150°C or less.

(4) Molding process Next, the clarified glass is supplied to a molding device and molded into a plate shape. Note that a stirring tank, a conditioning tank, etc. may be arranged between the clarification tank and the molding device, and the glass may be supplied to the molding device after passing through these. Furthermore, in order to prevent contamination of the glass, at least the contact surface with the glass of the communication channel connecting the melting furnace, clarification tank, and molding equipment (or each tank installed between them) must be made of platinum or platinum alloy. is preferred.

Although the molding method is not particularly limited, the effects of the present invention can be easily enjoyed by adopting the down-draw method, which is limited by the length of the lehr and makes it difficult to reduce the thermal shrinkage rate. As the down-draw method, it is preferable to employ an overflow down-draw method. The overflow down-draw method involves overflowing molten glass from both sides of a trough-shaped refractory with a wedge-shaped cross section, merging the overflowing molten glass at the lower end of the trough-shaped refractory, and stretching it downward to form the glass into a plate. This is a method of molding into shapes. In the overflow down-draw method, the surface of the glass substrate that is to become the surface does not come into contact with the trough-like refractory and is formed as a free surface. Therefore, an unpolished glass substrate with good surface quality can be manufactured at low cost, and it is also easy to make the glass larger and thinner. Note that the structure and material of the trough-like refractory used in the overflow down-draw method are not particularly limited as long as desired dimensions and surface accuracy can be achieved. Furthermore, the method of applying force when performing downward stretch molding is not particularly limited. For example, a method may be adopted in which a heat-resistant roll having a sufficiently large width is rotated while in contact with the glass, or a plurality of pairs of heat-resistant rolls are brought into contact only near the end surface of the glass. You may adopt the method of stretching. In addition to the overflow downdraw method, it is also possible to adopt, for example, a slot down method.

The glass thus formed into a plate shape is cut into a predetermined size, and subjected to various chemical or mechanical processing, etc., as necessary, to become a glass substrate.

(5) Evaluation step In the present invention, an evaluation step may be provided to evaluate the bubble quality of the obtained glass. It is preferable to adjust the bubble diameter expansion start temperature based on the evaluation results of the bubble quality in this step. For example, if the foam quality is below the standard, the temperature at which foam diameter expansion starts may exceed the maximum clarification temperature. In such a case, it is necessary to adjust the bubble diameter expansion start temperature. The bubble diameter expansion starting temperature can be adjusted by changing the conditions of the melting step (2). Specifically, the bubble diameter expansion starting temperature can be adjusted by changing the maximum temperature in the melting furnace (for example, in the case of electric melting without burner combustion, the temperature near the bottom surface).

Next, an alkali-free glass substrate that can be produced by the method of the present invention will be described. Note that the composition of the alkali-free glass substrate and the temperature at which bubble diameter expansion starts are as described above, and their explanation will be omitted here.

The alkali-free glass substrate obtained by the method of the present invention is obtained by heating the glass from room temperature to 500°C at a rate of 5°C/min, holding it at 500°C for 1 hour, and then cooling the glass at a rate of 5°C/min. When the heat shrinkage rate is 30 ppm or less, 28 ppm or less, 27 ppm or less, 26 ppm or less, 25 ppm or less, 24 ppm or less, 23 ppm or less, 22 ppm or less, 21 ppm or less, 20 ppm or less, 19 ppm or less, 18 ppm or less, 17 ppm or less, 16 ppm or less, especially 15 ppm. It is preferable that it is as follows. If the thermal shrinkage rate is large, it becomes difficult to use it as a substrate for forming low-temperature polysilicon TFTs.

The alkali-free glass substrate obtained by the method of the present invention has a β-OH value of 0.3/mm or less, 0.25/mm or less, 0.2/mm or less, 0.18/mm or less, and 0.16/mm or less. It is preferably made of glass having a particle diameter of 0.15/mm or less, particularly 0.15/mm or less. Note that the lower limit of the β-OH value is not limited, but it is preferably 0.01/mm or more, particularly 0.05/mm or more. If the β-OH value is large, the strain point of the glass will not be sufficiently high, making it difficult to significantly reduce the thermal shrinkage rate.

The alkali-free glass obtained by the method of the present invention has a strain point of 650°C or higher, 650°C or higher, 660°C or higher, 670°C or higher, 680°C or higher, 685°C or higher, 690°C or higher, 695°C or higher, especially 700°C. It is preferable that it is above. In this way, thermal shrinkage of the glass substrate can be easily suppressed in the manufacturing process of low-temperature polysilicon TFTs. If the strain point is too high, the temperature during molding or melting becomes too high, which tends to increase the manufacturing cost of the glass substrate. Therefore, the alkali-free glass obtained by the method of the present invention preferably has a strain point of 750°C or lower, 740°C or lower, particularly 730°C or lower.

The alkali-free glass substrate obtained by the method of the present invention has a temperature corresponding to 10 ^4.5 dPa・s of 1340°C or lower, 1335°C or lower, 1330°C or lower, 1325°C or lower, 1320°C or lower, especially 1315°C or lower. Preferably, it is made of some glass. When the temperature at 10 ^4.5 dPa·s becomes high, the temperature during molding becomes too high, and the manufacturing cost of the glass substrate tends to rise. If the temperature at 10 ^4.5 dPa·s is too low, it is difficult to design a high viscosity at the liquidus temperature. Therefore, it is preferable that the temperature corresponding to 10 ^4.5 dPa·s is 1190°C or higher, 1200°C or higher, particularly 1210°C or higher. Note that the "temperature equivalent to 10 ^4.5 dPa·s" is a value measured by the platinum ball pulling method.

The alkali-free glass substrate obtained by the method of the present invention is a glass whose temperature at 10 ^2.5 dPa・s is 1650°C or lower, 1640°C or lower, 1630°C or lower, 1620°C or lower, 1615°C or lower, particularly 1610°C or lower. It is preferable to consist of. When the temperature at 10 ^2.5 dPa·s increases, glass becomes difficult to melt, the manufacturing cost of the glass substrate increases, and defects such as bubbles are more likely to occur. If the temperature at 10 ^2.5 dPa·s is too low, it is difficult to design the viscosity at the liquidus temperature to be high. Therefore, it is preferable that the temperature corresponding to 10 ^2.5 dPa·s is 1490°C or higher, 1500°C or higher, particularly 1510°C or higher. Note that the "temperature equivalent to 10 ^2.5 dPa·s" is a value measured by the platinum ball pulling method.

The alkali-free glass obtained by the method of the present invention preferably consists of glass having a liquidus temperature of less than 1280°C, less than 1270°C, less than 1260°C, less than 1250°C, less than 1240°C, particularly less than 1230°C. In this way, devitrification crystals are less likely to occur during glass production, making it easier to prevent productivity from decreasing. Furthermore, since it becomes easier to mold using the overflow down-draw method, it becomes easier to improve the surface quality of the glass substrate, and the manufacturing cost of the glass substrate can be reduced. In view of the recent increase in the size of glass substrates and the increase in the definition of displays, it is of great significance to improve the devitrification resistance in order to suppress as much as possible devitrification substances that can cause surface defects. Note that the liquidus temperature is an index of devitrification resistance, and the lower the liquidus temperature is, the better the devitrification resistance is. "Liquidus temperature" is the glass powder that passes through a standard sieve of 30 mesh (500 μm) and remains on the 50 mesh (300 μm), and is placed in a platinum boat and placed in a temperature gradient furnace set at 1100°C to 1350°C for 24 hours. This refers to the temperature at which devitrification (crystalline foreign matter) is observed in the glass when the platinum boat is taken out after holding the glass.

The alkali-free glass substrate obtained by the method of the present invention has a viscosity at the liquidus temperature of 10 ^4.0 dPa·s or more, 10 ^4.1 dPa·s or more, 10 ^4.2 dPa·s or more, 10 ^4.3 dPa・s or more, 10 ^4.4 dPa・s or more, 10 ^4.5 dPa・s or more, 10 ^4.6 dPa・s or more, 10 ^4.7 dPa・s or more, 10 ^4.8 dPa・s or more, It is preferable that it is made of glass having a pressure of 10 ^4.9 dPa·s or more, particularly 10 ^5.0 dPa·s or more. In this way, devitrification is less likely to occur during molding, making it easier to mold the glass substrate using the overflow down-draw method.As a result, it becomes possible to improve the surface quality of the glass substrate, and it also becomes possible to manufacture the glass substrate. Costs can be reduced. Note that the viscosity at the liquidus temperature is an index of moldability, and the higher the viscosity at the liquidus temperature, the better the moldability. Note that "viscosity at liquidus temperature" refers to the viscosity of glass at liquidus temperature, and can be measured, for example, by a platinum ball pulling method.

[Example 1]
Hereinafter, embodiments of the manufacturing method of the present invention will be described. FIG. 2 is an explanatory diagram showing a schematic configuration of a suitable glass manufacturing facility 1 for carrying out the manufacturing method of the present invention.

First, the configuration of the glass manufacturing equipment will be explained. The glass manufacturing equipment 10 includes a melting kiln 1 that electrically melts raw material batches, a fining tank 2 provided downstream of the melting kiln 1, an adjustment tank 3 provided downstream of the clarification tank 2, and an adjustment tank 3 provided downstream of the clarification tank 2. The melting furnace 1, the clarification tank 2, the adjustment tank 3, and the molding device 4 are connected by communication channels 5, 6, and 7, respectively.

The melting furnace 1 has a bottom wall, side walls, and a ceiling wall, and each of these walls is made of high zirconia refractory such as ZrO ₂ electrocast refractory or dense zircon. The side wall is designed to have a thin wall thickness so that the refractory can be easily cooled. Further, a plurality of pairs of molybdenum electrodes are installed at the lower portions of the side walls and the bottom wall on both the left and right sides. Each electrode is provided with cooling means to prevent the electrode temperature from rising excessively. By applying electricity between the electrodes, it is possible to directly heat the glass. Note that in this embodiment, a burner (excluding a burner at the start of production) and a heater used during normal production are not provided.

The upstream side wall of the melting furnace 1 is provided with an inlet for raw materials supplied from a furnace front silo (not shown), and the downstream side wall is formed with an outlet. The melting kiln 1 and the clarification tank 2 are connected through a narrow communication channel 5 having an upstream end thereof.

The clarification tank 2 has a bottom wall, a side wall, and a ceiling wall, and each of these walls is made of a high zirconia refractory. Further, the communication channel 5 has a bottom wall, a side wall, and a ceiling wall, and each of these walls is also formed of a high zirconia refractory such as ZrO ₂ electrocast refractory. The fining tank 2 has a smaller volume than the melting furnace 1, and the inner wall surfaces of its bottom wall and side walls (at least the inner wall surface portions that come into contact with the molten glass) are lined with platinum or a platinum alloy, and the connecting flow The inner wall surfaces of the bottom wall and side walls of the channel 5 are also lined with platinum or a platinum alloy. In this clarification tank 2, the downstream end of the outflow passage 5 is opened in the upstream side wall. The fining tank 2 is a part where glass is mainly clarified, and fine bubbles contained in the glass are enlarged and floated by the fining gas released from the fining agent and removed from the glass.

An outflow port is formed in the downstream side wall of the clarification tank 2, and the clarification tank 2 and the adjustment tank 3 are connected via a narrow communication channel 6 having the outflow port at the upstream end. .

The adjustment tank 3 has a bottom wall, a side wall, and a ceiling wall, and each of these walls is made of a high zirconia refractory. Further, the communication channel 6 has a bottom wall, a side wall, and a ceiling wall, and each of these walls is also formed of a high zirconia refractory such as ZrO ₂ electrocast refractory. The inner wall surfaces of the bottom wall and side walls of the adjustment tank 3 (at least the inner wall surface portions that come into contact with the molten glass) are lined with platinum or a platinum alloy, and the inner wall surfaces of the bottom wall and side walls of the communication channel 7 are coated with platinum or a platinum alloy. They are also lined with platinum or platinum alloy. The adjustment tank 3 is a part that mainly adjusts the glass to a state suitable for molding, and gradually lowers the temperature of the molten glass to adjust the viscosity to a state suitable for molding.

An outflow port is formed in the downstream side wall of the adjustment tank 3, and the adjustment tank 3 and the molding device 4 are connected via a narrow communication channel 7 having the outflow port at the upstream end. .

The molding device 4 is a down-draw molding device, for example, an overflow down-draw molding device. Further, the inner wall surfaces of the bottom wall and side walls of the communication channel 7 are lined with platinum or a platinum alloy.

Note that the supply route in this embodiment refers to the connection channel 5 provided downstream of the melting oven to the communication channel 7 provided upstream of the molding device. In addition, here we have illustrated a glass manufacturing facility consisting of a melting furnace, a clarification tank, an adjustment tank, and a molding device, but for example, it is possible to provide a stirring tank between the adjustment tank and the molding device to stir and homogenize the glass. is also possible. Further, each of the above-mentioned equipment is shown as having platinum or a platinum alloy lined inside the refractory, but it goes without saying that equipment made of platinum or a platinum alloy itself may be used instead.

A method for manufacturing glass using the glass manufacturing equipment having the above configuration will be described.

First, glass raw materials (and glass cullet) are mixed and prepared to obtain a desired composition.

Subsequently, the prepared glass raw materials are put into a melting furnace 1, where they are melted and vitrified. In the melting furnace 1, a voltage is applied to the molybdenum electrode to heat the glass directly. In this embodiment, since radiant heating by burner combustion is not performed, moisture in the atmosphere does not increase, and the amount of moisture supplied from the atmosphere into the glass is significantly reduced. In addition, the temperature near the bottom of the melting furnace is adjusted so that the bubble diameter expansion start temperature is lower than the maximum fining temperature.

In this embodiment, at the start of production, a burner is used to heat the glass raw material, and when the initially charged glass raw material becomes molten, the burner is stopped and the process shifts to direct current heating.

The molten glass vitrified in the melting furnace 1 is guided to the clarification tank 2 through the communication channel 5. Molten glass contains many bubbles caused by gas generated during the vitrification reaction and bubbles caused by air that exists between raw material particles and is trapped during vitrification. These bubbles are expanded and floated by the clarifying gas released from SnO ₂ , which is a clarifier component, and removed.

The molten glass clarified in the clarification tank 2 is guided to the adjustment tank through the communication channel 6. The molten glass guided to the conditioning tank 3 has a high temperature and low viscosity, and cannot be directly molded by a molding device. Therefore, the temperature of the glass is lowered in the adjustment tank 3, and the viscosity is adjusted to a value suitable for molding.

The molten glass whose viscosity has been adjusted in the adjustment tank 3 is led to an overflow down-draw molding device through a communication channel 7, and is molded into a thin plate shape. Further, cutting, end face processing, etc. are performed to obtain a glass substrate made of alkali-free glass.

According to the above method, it is possible to reduce the amount of water supplied into the glass as much as possible, so it is possible to reduce the β-OH value to 0.3/mm or less, and it is possible to produce glass with a small thermal shrinkage rate. Obtainable.

[Example 2]
Next, glass manufactured using the method of the present invention will be explained. Tables 1 to 5 show Examples (Nos. 1 to 6, 9 to 38) of the present invention and Comparative Examples (Nos. 7, 8, 39, 40).

まず表１～５の組成となるように珪砂、酸化アルミニウム、無水ホウ酸、炭酸カルシウム、硝酸ストロンチウム、炭酸バリウム、メタリン酸アルミ、酸化第二錫、塩化ストロンチウム、塩化バリウムを混合し、調合した。なおＮｏ．１～５については、さらに目標組成と同じ組成のガラスカレット（β－ＯＨ値 ０．１／ｍｍ、原料バッチの総量に対して３５質量％使用）を併用した。

First, silica sand, aluminum oxide, boric anhydride, calcium carbonate, strontium nitrate, barium carbonate, aluminum metaphosphate, tin oxide, strontium chloride, and barium chloride were mixed and prepared to have the compositions shown in Tables 1 to 5. Furthermore, No. For Nos. 1 to 5, glass cullet having the same composition as the target composition (β-OH value 0.1/mm, used at 35% by mass based on the total amount of the raw material batch) was used in combination.

Next, the glass raw materials were supplied to an electric melting kiln without burner combustion and melted, and then the molten glass was refined and homogenized in a refining tank and an adjustment tank, and the viscosity was adjusted to a value suitable for molding. The bubble diameter expansion starting temperature was adjusted by adjusting the temperature near the bottom of the melting oven. The maximum temperature during the process was set to be the highest in the clarification tank. In addition, the maximum temperature in the clarification tank was set to the temperature shown in each table. The maximum temperature in the clarification tank was confirmed by monitoring the temperature of platinum or platinum alloy attached to the inner wall of the clarification tank.

Subsequently, the molten glass was supplied to an overflow down-draw molding device, molded into a plate shape, and then cut to obtain a glass sample with a thickness of 0.5 mm. The molten glass leaving the melting kiln was supplied to the forming apparatus while coming into contact only with platinum or platinum alloy.

The obtained glass sample was evaluated for β-OH value, strain point, thermal shrinkage rate, bubble diameter expansion start temperature, and bubble quality. The results are shown in Tables 1 and 2.

As is clear from Tables 1 to 4, when the bubble diameter expansion initiation temperature was lower than the maximum fining temperature, excellent foam quality was obtained.
Table 5 also shows that when the difference between the bubble diameter expansion start temperature and the maximum fining temperature is sufficiently large, excellent foam quality can be obtained even when the flow rate is changed. On the other hand, when the bubble diameter expansion start temperature exceeded the maximum fining temperature, the bubble quality was poor regardless of the flow rate.

Note that the β-OH value of the glass was determined by measuring the transmittance of the glass using FT-IR and using the following formula.

β-OH value = (1/X)log10(T ₁ /T ₂ )
X: Glass thickness (mm)
T ₁ : Transmittance (%) at reference wavelength 3846 cm ^-1
T ₂ : Minimum transmittance (%) near hydroxyl group absorption wavelength 3600 cm ^-1

Strain points were measured based on the ASTM C336-71 method.

Thermal shrinkage rate was measured by the following method. First, as shown in FIG. 3(a), a strip-shaped sample G of 160 mm x 30 mm was prepared as a sample of the glass substrate 1. Markings M were formed on both ends of this strip-shaped sample G in the long side direction using #1000 waterproof abrasive paper at positions 20 to 40 mm away from the edges. Thereafter, as shown in FIG. 3(b), the strip-shaped sample G on which the marking M was formed was broken into two along the direction perpendicular to the marking M to produce sample pieces Ga and Gb. Then, only one sample piece Gb was heated from room temperature (25°C) to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then heat treated to be cooled down to room temperature at a rate of 5°C/min. . After the above heat treatment, as shown in FIG. 3(c), the markings M on the two sample pieces Ga and Gb are arranged in parallel with the sample piece Ga that has not been heat treated and the sample piece Gb that has been heat treated. The amount of positional shift (ΔL1, ΔL2) was read using a laser microscope, and the thermal shrinkage rate was calculated using the following formula. Note that l ₀ in the formula is the distance between the initial markings M.

Heat shrinkage rate = [{ΔL ₁ (μm) + ΔL ₂ (μm)}×10 ³ ]/l ₀ (mm) (ppm)
The bubble diameter expansion start temperature was determined as follows. First, the obtained glass plate was crushed and classified to 2.0 to 5.6 mm. 15 g of the classified glass was placed in a quartz tube and held at 1500° C. for 10 minutes. Thereafter, the temperature was raised from 1500°C at a heating rate of 2°C/min, and the behavior of bubbles in the glass melt was photographed with a video camera. Using the captured video or observation images extracted from the video, three or more bubbles with a diameter of 100 μm or less were selected and their bubble diameters were measured at every 10°C. Based on this observation result, the temperature at which the bubble diameter expansion length was 50 μm or more was defined as the bubble diameter expansion start temperature.

Foam quality is determined by counting bubbles with a diameter of 100 μm or more, and ``◎'' if the number is 0.05 or less, ``〇'' if the number is 0.05 to 0.1, or 0.1. When it was ~0.3 pieces/kg, it was displayed as "△", and when it exceeded 0.3 pieces/kg, it was displayed as "×".
The molding flow rate refers to the flow rate of molten glass entering the molding device 4 from the flow path 7 in FIG. The ratio of "molding flow rate of No. X/molding flow rate of No. 1" is shown based on 1.

According to the present invention, it is possible to easily obtain a glass substrate that has good bubble quality and a low thermal shrinkage rate suitable for manufacturing low-temperature polysilicon TFTs.

1 Melting furnace 2 Clarifying tank 3 Adjustment layer 4 Molding devices 5, 6, 7 Communication channel 10 Glass manufacturing equipment

Claims (11)

Glass composition, in mass%, SiO ₂ 50-70%, Al ₂ O ₃ 15-25%, B ₂ O ₃ more than 4.5-12%, MgO 0-10%, CaO 0-15%, SrO 0 -10%, BaO 0-15%, ZnO 0-5%, ZrO ₂ 0-5%, TiO ₂ 0-5%, P ₂ O ₅ 0-15%, SnO ₂ 0-0.5% A batch preparation process in which a raw material batch is prepared to become alkali-free glass, a melting process in which the prepared raw material batch is melted by electric melting by direct current heating, a fining process in which the molten glass is clarified, and the clarified glass is A molding step of molding into a plate shape,
The raw material batch is melted by adjusting the temperature at the bottom of the melting kiln so that the temperature at which the resulting glass bubbles begin to expand is lower than the maximum temperature in the fining process, and radiant heating by burner combustion is not used in the melting process. A method for manufacturing an alkali-free glass substrate, characterized by: including an evaluation step of evaluating the bubble quality of the obtained glass, and based on the evaluation result of the bubble quality in the evaluation step, the temperature at which the bubble diameter expansion starts of the obtained glass is lower than the maximum temperature in the fining step. 2. The method for manufacturing an alkali-free glass substrate according to claim 1, wherein the raw material batch is melted by adjusting the temperature of the bottom surface in the melting furnace. The method for producing an alkali-free glass substrate according to claim 1 or 2, characterized in that the raw material batch is melted so that the temperature at which bubble diameter expansion of the resulting glass starts is 1520 to 1680°C. The method for producing an alkali-free glass substrate according to any one of claims 1 to 3 , characterized in that chloride is added to the raw material batch. The method for producing an alkali-free glass substrate according to any one of claims 1 to 4 , characterized in that boric anhydride is used as at least a part of the glass raw material serving as a boron source. The method for producing an alkali-free glass substrate according to any one of claims 1 to 5 , characterized in that the raw material batch does not contain a hydroxide raw material. A method of manufacturing an alkali-free glass substrate by adding glass cullet to a raw material batch, the method comprising using glass cullet made of glass with a β-OH value of 0.4/mm or less as at least a part of the glass cullet. The method for manufacturing an alkali-free glass substrate according to any one of claims 1 to 6 . The alkali-free glass substrate according to any one of claims 1 to 7 , characterized in that glass raw materials and/or melting conditions are adjusted so that the β-OH value of the obtained glass is 0.3/mm or less. manufacturing method. The method for producing an alkali-free glass substrate according to any one of claims 1 to 8 , characterized in that the strain point of the glass obtained is higher than 650°C. 10. The method for producing alkali-free glass according to claim 1, wherein the resulting glass has a thermal shrinkage rate of 30 ppm or less. A batch preparation step of preparing a raw material batch to obtain an aluminosilicate-based alkali-free glass having a B ₂ O ₃ content of more than 4.5 to 12% by mass, and melting the prepared raw material batch by electric melting by direct current heating. A fining process of fining the molten glass, and a forming process of molding the clarified glass into a plate shape,
The raw material batch is melted by adjusting the temperature at the bottom of the melting kiln so that the temperature at which the resulting glass bubbles begin to expand is lower than the maximum temperature in the fining process, and radiant heating by burner combustion is not used in the melting process. A method for manufacturing an alkali-free glass substrate, characterized by:

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